Improve PDF / AI (Adobe Illustrator) recognition (#396)

Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>

core.js

@@ -55,15 +55,6 @@ function _check(buffer, headers, options) {
 	return true;
 }
 
-async function _checkSequence(sequence, tokenizer, ignoreBytes) {
-	const buffer = Buffer.alloc(minimumBytes);
-	await tokenizer.ignore(ignoreBytes);
-
-	await tokenizer.peekBuffer(buffer, {mayBeLess: true});
-
-	return buffer.includes(Buffer.from(sequence));
-}
-
 async function fromTokenizer(tokenizer) {
 	try {
 		return _fromTokenizer(tokenizer);
@@ -79,7 +70,6 @@ async function _fromTokenizer(tokenizer) {
 	const bytesRead = 12;
 	const check = (header, options) => _check(buffer, header, options);
 	const checkString = (header, options) => check(stringToBytes(header), options);
-	const checkSequence = (sequence, ignoreBytes) => _checkSequence(sequence, tokenizer, ignoreBytes);
 
 	// Keep reading until EOF if the file size is unknown.
 	if (!tokenizer.fileInfo.size) {
@@ -596,9 +586,13 @@ async function _fromTokenizer(tokenizer) {
 	}
 
 	if (checkString('%PDF')) {
+		await tokenizer.ignore(1350);
+		const maxBufferSize = 10 * 1024 * 1024;
+		const buffer = Buffer.alloc(Math.min(maxBufferSize, tokenizer.fileInfo.size));
+		await tokenizer.readBuffer(buffer, {mayBeLess: true});
+
 		// Check if this is an Adobe Illustrator file
-		const isAiFile = await checkSequence('Adobe Illustrator', 1350);
-		if (isAiFile) {
+		if (buffer.includes(Buffer.from('AIPrivateData'))) {
 			return {
 				ext: 'ai',
 				mime: 'application/postscript'

test.js

@@ -181,6 +181,17 @@ const names = {
 		'fixture',
 		'fixture.wma',
 		'fixture.wmv'
+	],
+	ai: [
+		'fixture-normal', // Normal AI
+		'fixture-without-pdf-compatibility' // AI without the PDF compatibility (cannot be opened by PDF viewers I guess)
+	],
+	pdf: [
+		'fixture',
+		'fixture-adobe-illustrator', // PDF saved from Adobe Illustrator, using the default "[Illustrator Default]" preset
+		'fixture-smallest', // PDF saved from Adobe Illustrator, using the preset "smallest PDF"
+		'fixture-fast-web', // PDF saved from Adobe Illustrator, using the default "[Illustrator Default"] preset, but enabling "Optimize for Fast Web View"
+		'fixture-printed' // PDF printed from Adobe Illustrator, but with a PDF printer.
 	]
 };